This invention concerns dicarboxylates derived from sulfonated lignin material, the process for making said derivatives and, more particularly, the use of such dicarboxylates for metal complexing.
As used herein, the term "lignin" has its normal connotation, and refers to the substance which is typically recovered from alkali pulping black liquors, such as are produced in the kraft, soda and other well-known alkali pulping operations. The term "sulfonated lignin", as used in this specification, refers to the product which is obtained by the introduction of sulfonic acid groups into the lignin molecule, as may be accomplished by reaction of the lignin with sulfite or bisulfite compounds. As used herein, the term "lignosulfonate" refers to the reaction product of lignin which is inherently obtained during the sulfite pulping of wood, straw, corn stalks, bagasse and the like, and is a principal constituent of the spent sulfite liquor which is derived from that process. Finally, the phrase "sulfonated lignin material" encompasses not only the lignin and lignosulfonate reaction products herein above described, but also spent sulfite liquors, which materials may be further reacted, purified, fractionated, or the like, as may be appropriate for the particular material involved.
Numerous modifications have been made to sulfonated lignin material in order to improve certain properties, such as dispersing ability, flocculating ability, flame retardancy, thermoplasticity, tanning properties and the like. Although lignin derivatives with improved properties have been prepared by treatment of lignin, none of these derivatives has proven to have as great effectiveness in both dispersing the metal complexing properties as is desirable. This is particularly true with regards to the sequestering capacity of lignin for calcium, copper, iron, magnesium, manganese, zinc and other metals commonly encountered in the industry. For example, the catecholic groups in lignin and ferrous ions are known to form a blue color complex and a colorimetric technique based on the complex formation has been commonly used to determine the amount of catecholic structures in kraft lignin (see Falkehag et al. 1966 in "Lignin Structure and Reaction", Advances in Chemistry Series 59:75). U.S. Pat. No. 3,956,261 reveals a lignodicarboxylic acid made by reacting an alkali lignin or sulfonated lignin with epoxysuccinate which is claimed to be an effective metal complexing agent. Commercial lignin products which combine lignosulfonate with metal ions, such as Fe, Zn, Mg, Mn, are sold as micronutrients for agricultural applications. The amounts of complexed metals in the products are generally low. Some lignin sequestrants also exhibit the ability to enhance corrosion inhibition of certain industrial zinc formulations.
Polycarboxylic compounds are widely used as calcium complexing agents in detergent building, water treatment, metal cleaning and other applications. Among them, the most important are citric acid, ethylenediaminotetraacetic acid (EDTA) and nitrilotriacetic acid (NTA). A general structural feature of these compounds is that the polycarboxylate groups in a molecule are coupled with an electron-rich element, such as oxygen or nitrogen, in the manner depicted in the formula ##STR2## where R.sub.1, R.sub.2 =hydrogen, alkyl or aliphatic carboxylic group; and X=oxygen or nitrogen. Another common feature of these compounds is that they all lack a sulfonic acid group which is required to give water solubility under strongly acidic conditions or at high water hardness.
The invention described herein is a ligno-aminodicarboxylic acid of strong metal complexing ability and good water solubility. It is a derivative of sulfonated lignin material which is abundantly available from the spent liquors of wood pulping processes. By virtue of its unique performance characteristics, the product is particularly useful as a complexing agent for formulating concentrated metal solutions.
It is, therefore, the general object of this invention to provide a sulfonated ligno-aminodicarboxylic acid.
Another specific object of this invention is to provide a sulfonated lignin derivative which exhibits a strong metal complexing ability and good water solubility.
Still another object of this invention is to provide an effective process for producing the sulfonated ligno-aminodicarboxylic acid.
Other objects, features and advantages of this invention will be evident from the following detailed description.